Economizer bypass with ammonia injection

ABSTRACT

A NOx removal system for a furnace or boiler with an economizer has two separate ammonia injection grids (AIG) upstream of an SCR. A primary AIG is positioned adjacent the flue gas outlet of the economizer and upstream of an economizer bypass outlet. A secondary AIG is located within an economizer flue gas bypass. The separate AIGs permit a reduction in the overall length of the flue between the economizer and SCR, thereby reducing the NOx removal system footprint, cost and quantity of materials.

FIELD SAND BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to the field of utilityboilers and furnaces, and in particular to a new and useful SelectiveCatalytic Reduction (SCR) installation in which separate ammoniainjection grids (AIGs) are located in each of the economizer outlet flueand economizer flue gas bypass of a boiler for control of NOx emissions.

[0002] NOx refers to the cumulative emissions of nitric oxide (NO),nitrogen dioxide (NO₂) and trace quantities of other species generatedduring combustion of fossil fuel in boilers and furnaces. Conversion ofmolecular and fuel nitrogen into NOx is promoted by high temperaturesand high volumetric heat release rates found in boilers.

[0003] SCR systems are used to clean impurities from the exhaust gasesof boilers and furnaces, and in particular, to reduce NOx emissions.Ammonia is injected into the boiler exhaust gas stream. Ammonia istypically introduced using sparger tubes to spray ammonia into theexhaust gas stream. The sparger tubes form an ammonia injection grid(AIG). A chemical reaction occurs with the exhaust gases in the presenceof an SCR catalyst, which removes a large portion of NOx from theexhaust gases and converts it to water and elemental nitrogen.

[0004] Additional details of SCR systems for NOx removal are provided inChapter 34 of Steam/its generation and use, 40th Edition, Stultz andKitto, Eds., Copyright® 1992, The Babcock & Wilcox Company, the text ofwhich is hereby incorporated by reference as though fully set forthherein.

[0005] It is desirable to maintain the temperature of flue gases leavingthe last heat trap of a boiler entering an SCR at or above the NO_(x)reduction catalyst's minimum operating temperature. The minimumtemperature must be maintained even as the boiler load is reduced.

[0006] Presently, a preferred method for maintaining the flue gastemperature above the SCR operating minimum is to install an economizerbypass duct system. The economizer bypass duct transports flue gasesfrom upstream of the last heat trap, or economizer, to just prior toentry into the NO_(x) reduction device—the SCR. The diverted flue gasflowing the through the bypass has a higher temperature since it did notpass over the economizer heat exchange surfaces. Thus, the diverted fluegas raises the temperature of the flue gases entering the SCR above theminimum operating temperature.

[0007] The flow of flue gases through the bypass is controlled usingdampers in the economizer outlet and in the bypass. By opening orclosing the dampers, the percentage flow through the bypass is easilyadjusted.

[0008] However, it has been found that in certain new boiler sites, orwhere existing boilers are being retrofitted with SCR systems, thatthere is insufficient space to install an economizer bypass duct in acost-effective manner. In such cases, either there would be no controlover the temperature of the flue gases entering the SCR, or theinstallation of a bypass would be so costly as to be prohibitive ofmaking the modification.

[0009] As the catalysts used in SCR systems are carefully engineered andexpensive, it is beneficial to be able to control the stoichiometry andtemperature of the exhaust gas/ammonia/catalyst reaction. Insufficienttemperature of the flue gases in the SCR where the ammonia is injectedwill result in salt formation of fouling ammonia salts in the flues.

[0010] An ammonia injection grid (AIG) design presently in use hasmultiple levels of sparger tubes having spray openings. The AIG islocated within a single designated space, positioned downstream of theeconomizer and bypass outlets. The spray openings are oriented parallelwith the exhaust gas flow, toward the catalyst. The position of the AIGis selected to avoid spraying ammonia into portions of the flue thatwill have too low a temperature during bypass mode operation, so as toprevent formation of ammonia salts in the flue. Groupings of the tubesare supplied by independently controlled supply headers to create zoneswithin the grid.

[0011] While the grid design permits greater control over the dispersionof ammonia into the exhaust gas stream, it also results in blockage of alarge area of the downstream exhaust gas flow path. A large number oftubes and pipes are needed to provide ammonia to each zone of the grid,which creates the blockage. The blockage in turn results in a pressuredrop between the furnace side and exhaust side. The pressure drop is notdesirable as it adversely affects the overall efficiency of the boileror furnace system.

[0012] And, to ensure proper flue gas mixing and velocity, the fluelength must be a minimum length both up and downstream of the AIG. Thus,locating the AIG downstream of the bypass outlet results in a necessaryextension of the flue length, further resulting in increased footprint,cost and materials.

[0013] Current consensus with respect to the operation of SCR's is toprovide the highest level of flow with temperature and chemicalcomponent uniformity. Techniques such as zonal AIG injection, staticmixing and judicious use of flow correcting devices have all been usedin known SCR designs to attempt to provide the most uniform flow andblend composition as practical. However, these techniques still do notprovide a perfectly uniform flow or blend, so that improvement ispossible.

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a NOx removalsystem for furnaces and boilers which is more compact than currentsystems, while preventing the formation of fouling ammonia salts.

[0015] It is a further objection of the invention to provide an SCRinstallation for furnaces and boilers which is less costly than currentsystems while providing the same or better NOx removal from flue gases.

[0016] Accordingly, an SCR installation of the invention for a furnaceor boiler having an economizer and economizer bypass has two ammoniainjection grids. A primary AIG is located between the economizer outletand the economizer bypass outlet, while a secondary AIG is positionedwithin the bypass duct. Placing the AIGs in these locations permits theuse of the fluework for flue gas velocity distribution and mixing offlue gases and ammonia in a shorter overall flue length. The bypass andeconomizer are each provided with dampers for controlling the flow offlue gases through the respective ducts. The ammonia injection grids canbe controlled to provide ammonia in proportion to the flow of gasesthrough the economizer and bypass as well.

[0017] The various features of novelty which characterize the inventionare pointed out with particularity in the claims annexed to and forminga part of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawing and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The sole FIGURE is a side elevation schematic view of aneconomizer and SCR reactor of a furnace or boiler in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Referring now to the drawing, a NOx removal system 10 of theinvention is shown used in an economizer section 50 of a furnace orboiler. The economizer section 50 includes economizer 52 and economizerbypass duct 54. An SCR reactor 60 is located at the outlet end of theflue gas duct 58. Two separate ammonia injection grids (AIG) 20, 30 areprovided in the economizer section 50.

[0020] Primary AIG 20 is positioned in economizer flue 56 betweeneconomizer flue gas outlet 55 and bypass duct outlet 57. Secondary AIG30 is located near the upper end of economizer bypass duct 54. Each ofthe primary and secondary AIGs 20, 30 is positioned downstream ofcontrol dampers 40, 42. Bypass control dampers 40 are located adjacentthe economizer bypass duct inlet 53. Economizer dampers 42 are arrangedin the economizer flue 56 just upstream of the primary AIG 20. Theprimary and secondary AIGs 20, 30 are connected to ammonia supply andcontrol 80, which is operated to control the delivery of ammonia througheach AIG 20, 30.

[0021] As in known NOx removal systems, the dampers 40, 42 are adjustedbetween opened and closed to control the flow of flue gases through theeconomizer bypass duct 54 versus the economizer 52. That is, thepercentage of the flow of flue gases through the economizer bypass duct54 can be varied between 0-100% inclusive.

[0022] During non-bypass mode, when the flue gases all pass through theeconomizer 52, ammonia is only provided to the primary AIG 20. As fluegases are directed through the economizer bypass duct 54, some of theammonia is directed to the secondary AIG 30, so that the flow to primaryAIG 20 is reduced. Alternatively, when flue gases are directed primarilythrough the economizer bypass duct 54, the AIGs 20, 30 can be operatedto only provide ammonia to the economizer bypass duct 54. The formationof fouling ammonia salts due to low temperature in the economizer 52 isprevented by directing the appropriate portion of the ammonia from thesupply and control 80 to the secondary AIG 30.

[0023] Flue gas velocity distribution and ammonia/flue gas mixing duringthe bypass mode of operation is not as critical compared to non-bypassmode due to increased flue gas retention time in the SCR 60 at low heatinput/flue gas flow. Therefore, the number of ammonia injection pointsin the secondary AIG 30 and the length of the economizer bypass duct 54upstream and downstream of the secondary AIG 30 are not as criticalcompared with the same parameters in the primary AIG 20. Thus, thesecondary AIG 30 may be constituted by a single set of injection pipes,a set of injection pipes with multiple openings for ammonia injection oreven a single pipe with one or more injection holes. The pipes may bearranged in an array of parallel pipes with the injection openingsoriented directly upstream, or at an angle to the travel of flue gases.

[0024] During bypass mode operation when flue gas is passed through boththe economizer 52 and economizer bypass duct 54, injection of a portionof the ammonia at each of the primary AIG 20 and secondary AIG 30 willresult in a better ammonia to NOx distribution at the SCR reactor 60entrance. The ammonia has sufficient time, even in a shortened flue gasduct 58 between the economizer 52 and SCR reactor 60, to mix anddistribute within the flue gases. Further, introduction of ammonia tothe flue gases in the economizer bypass duct 54 prevents introduction oflarge amounts of ammonia-less flue gases to the flue gas stream at thebypass duct outlet 57.

[0025] As a result, the length of the flue gas duct 58 can be reduced byup to about 50% of the length of conventional NOx removal systems. Theduct length reduction translates to significant cost savings and smallerfootprint furnaces and boilers. The invention permits these savingswithout sacrificing the efficiency or quality of NOx removal from fluegases.

[0026] While not shown in the drawing, static mixing devices positioneddownstream of the secondary ammonia injection grid 30 can be used toimprove the mixing and ammonia distribution efficiency within theeconomizer bypass duct 54.

[0027] While specific embodiments and/or details of the invention havebeen shown and described above to illustrate the application of theprinciples of the invention, it is understood that this invention may beembodied as more fully described in the claims, or as otherwise known bythose skilled in the art (including any and all equivalents), withoutdeparting from such principles.

We claim:
 1. A NOx removal system for a furnace or boiler having aneconomizer with a flue gas bypass duct for bypassing flue gas around theeconomizer, the bypass duct extending between a bypass duct inletlocated upstream of the economizer and a bypass duct outlet downstreamof an economizer flue gas outlet, the bypass duct outlet being upstreamof a selective catalytic reduction reactor, the system comprising: aprimary ammonia injection grid positioned between the economizer fluegas outlet and the bypass duct outlet; and a secondary ammonia injectiongrid positioned within the flue gas bypass duct.
 2. A system accordingto claim 1, further comprising ammonia control means for controlling theamount of an ammonia supply injected through each of the primary andsecondary ammonia injection grids.
 3. A system according to claim 2,further comprising first dampers adjacent the economizer outlet andsecond dampers adjacent the bypass duct inlet.
 4. A system according toclaim 3, wherein the primary ammonia injection grid is positioneddownstream of the first dampers.
 5. A system according to claim 3,wherein the secondary ammonia injection grid is positioned downstream ofthe second dampers.
 6. A system according to claim 3, wherein the firstand second dampers are adjustable to direct from 0-100% of the fluegases through the bypass duct.
 7. A system according to claim 6, whereinthe ammonia control means is adjustable to direct from 0-100% of theammonia through the secondary ammonia injection grid.
 8. A systemaccording to claim 1, wherein the secondary ammonia injection gridcomprises at least one injection pipe.
 9. A system according to claim 1,wherein the secondary ammonia injection grid comprises a plurality ofinjection pipes each having a plurality of injection holes.
 10. Anammonia injection system for introducing ammonia to a flue gas stream ofa furnace or boiler having an economizer and an economizer flue gasbypass upstream of a selective catalytic reduction reactor, the systemcomprising: a primary ammonia injection grid positioned between a fluegas outlet of the economizer and a bypass outlet; a secondary ammoniainjection grid located in the economizer flue gas bypass; and controlmeans for controlling a percentage of an ammonia supply delivered toeach of the primary and secondary ammonia injection grids.
 11. A systemaccording to claim 10, wherein the control means is adjustable todeliver from 0-100% of the ammonia supply through the secondary ammoniainjection grid.
 12. A system according to claim 10, wherein thesecondary ammonia injection grid comprises at least one injection pipe.13. A system according to claim 10, wherein the secondary ammoniainjection grid comprises a plurality of injection pipes each having aplurality of injection holes.